1. Field of the Invention
The present invention relates to an electronic control timer for a microwave oven, and more particularly to an electronic control timer for a microwave oven which is capable of setting a total cooking period by various time units, has a simple internal structure and can be manufactured at a low cost.
2. Description of the Prior Art
Various types of some microwave ovens having an automatic cooking function are well known in the art. FIG. 8 is a view for illustrating a conventional microwave oven 10. Generally, microwave oven 10 includes a microprocessor 20 for controlling the operation of microwave oven 10, a power supply unit 30 and a magnetron 40 for generating microwaves. Further, microwave oven 10 includes an in-flow air temperature sensor 60a, an out-flow air temperature sensor 60b, a weight sensing sensor 60c, an in-flow air temperature sensing circuit 62a, an out-flow air temperature sensing circuit 62b, weight sensing circuit 62c, first, second and third analog/digital converters 64a, 64b and 64b, a cooking chamber 70, a turntable 72, a turntable motor 74 for rotating turntable 72, a cooling fan 80a, a blasting fan 80b, a cooling fan motor 82a and a blasting fan motor 82b.
FIG. 9 is a perspective view of the microwave oven 10 having the above structure. Microwave oven 10 includes a housing 12 having an opening at the front thereof, a door 14 and an operation panel 50. Door 14 is pivotally mounted to one side end of the housing 12 in order to close the front opening of the housing 12. Operation panel 50 is positioned at the right front portion of housing 12. Operation panel 50 includes a cooking function selection switch 52 and a timer 54. Cooking function selection switch 52 controls the output of microwaves for heating and cooking food within cooking chamber 70 of microwave oven 10. Timer 54 sets the cooking period of the food. On operation panel 50, a symbol for selecting the cooking function which corresponds to the amount of microwaves being output is displayed around cooking function selection switch 52. The cooking period which can be set is displayed around timer 54. A door switch 56 for opening and closing the door 14 is positioned below the timer 54.
Hereinbelow, the operation of conventional microwave oven 10 will be described with reference to FIGS. 8 and 9 in detail.
The food to be cooked is first inserted into cooking chamber 70. When a user simultaneously sets the cooking period by means of timer 54 and the cooking function by means of selection switch 52, an electric signal corresponding to the numerical value established by timer 54 and selection switch 52 is sent to microprocessor 20. Based on the electric signal, microprocessor 20 operates power supply unit 30.
Power supply unit 30 simultaneously supplys electric power to magnetron 40, turntable motor 74, cooling fan motor 82a and blasting fan motor 82b. When the electric power is supplied from power supply unit 30, magnetron 40 generates microwaves for cooking the food. Cooling fan motor 82a actuates cooling fan 80a for cooling magnetron 40 so that cooling fan 80a aircools magnetron 40. Also, blasting fan motor 82b actuates blasting fan 80b for introducing the air into cooking chamber 70 so that blasting fan 80b blows the air into cooking chamber 70. As a result, the air temperature in cooking chamber 70 can be constantly maintained. Further, turntable motor 74 rotates turntable 72 on which the food to be cooked has been put so it is cooked uniformly.
Meantime, when the temperature equilibrium in cooking chamber 70 is established after the predetermined cooking period has elapsed, in-flow air temperature sensor 60a and out-flow air temperature sensor 60b which are respectively mounted on an inlet 84a and an outlet 84b of cooking chamber 70 detect the temperatures of the air and generate temperature sensing signals. The temperature sensing signals which have been generated from in-flow air temperature sensor 60a and out-flow air temperature sensor 60b are sent to first and second analog/digital converter 64a and 64b via in-flow air temperature sensing circuit 62a and out-flow air temperature sensing circuit 62b. First and second analog/digital converters 64a and 64b convert the analog signals which are the temperature sensing signals into digital signals, and send the digital signals to microprocessor 20. Then, microprocessor 20 decides the temperature increment of cooking chamber 70 based on the temperature sensed by in-flow air temperature sensor 60a and out-flow air temperature sensor 60b.
Meanwhile, weight sensing sensor 60c which is diposed below the outside bottom surface of cooking chamber 70 senses the weight of the food after the predetermined cooking period has elapsed and generates a weight sensing signal. The weight sensing signal as generated above is sent to third analog/digital converter 64c via weight sensing circuit 62c. Third analog/digital converter 64c converts an analog signal which is the weight sensing signal into a digital signal, and sends the digital signal to microprocessor 20. Microprocessor 20 determines the most suitable cooking state based on the weight sensing signal. That is, microprocessor 20 determines the appropriate heating time by discriminating the change in weight depending on the progress of the cooking period, and controls magnetron 40 on this basis.
In the conventional microwave oven 10 which operates as above, timer 54 for setting the cooking period has a mechanical structure capable of setting the cooking period based on the rotating angular of a timer knob.
Generally, timer 54 is designed so as to set the cooking period in such a manner that timer 54 can define the cooking period by using a time unit of one minute in relation to a cooking period of zero to ten minutes and a time unit of five minutes in relation to a cooking period of ten to thirtyfive minutes. Therefore, timer 54 has the mechanical structure for controlling a first timing section corresponding to the cooking period of zero to ten minutes and a second timing section corresponding to the cooking period of ten to thirtyfive minutes. In order to control the first and second timing sections, it is necessary to have a plurality of gears and a plurality of synchro-motors for rotating the gears. Further, it is necessary to have a control device for controlling the gears and the synchro-motors, and a plurality of electric wires for electrically connecting the gears with synchro-motors. Therefore, conventional timer 54 has a complicated internal structure. As a result, a manufacturing procedure of timer 54 can be complicated, and the manufacturing cost of timer 54 increases.
U.S. Pat. No. 5,107,088 issued to Masayuki Aoki on Apr. 21, 1992 discloses a timer capable of setting the cooking period and cooking start by use of a single timer knob.
FIGS. 10 and 11 illustrate Masayuki Aoki's timer 54a. Timer 54a includes a timer knob 102, a switch case 106, a switch shaft 108, a contact disc 110 and a switch substrate 112. Timer knob 102 and switch case 106 are mounted on an operation panel 50a by a nut 104. Switch shaft 108 is rotatably mounted on switch case 106. One end of switch shaft 108 is outwardly projected through operational panel 50a. Timer knob 102 is coupled with one end of switch shaft 108 so as to be rotated therewith. A contact arm 120 having first, second and third contacts 120a, 120b and 120c and a switch substrate 112 having a printed circuit board are attached to switch case 106.
Switch substrate 112 has a common conductive pattern 124, a first conductive pattern 126 and a second conductive pattern 128 which concentrically printed on one surface thereof. A plurality of scan points or pulse generating conductors 126a are printed at the inner periphery of the first conductive pattern 126 at equal pitches. Also, a plurality of scan points or pulse generating conductors 128a are printed on the outer periphery of the second conductive pattern 128 at the same pitch as pulse generating conductors 126a. Pulse generating conductors 128a are shifted slightly relative to pulse generating conductors 126a in a clockwise direction.
A plurality of groove-like contact stops 118 are formed on switch substrate 112. On switch substrate 112, contact stops 118 are radially formed in order to be positioned between one of pulse generating conductors 126a and an adjacent one thereof and between one of pulse generating conductor 128a and an adjacent one thereof. First, second and third contacts 120a, 120b and 120c of contact arm 120 are positioned on one of contact stops 118. First contact 120a is brought into contact with common conductive pattern 124. Second contact 120b is brought into contact with a portion of a surface of switch substrate 112a between pulse generating conductors 126a. Also, third contact 120c is brought into contact with a portion of the surface of the switch substrate 112a between pulse generating conductors 128a. First, second and third lead terminals 114, 116 and 118 on switch substrates 112 are electrically connected to common conductive patterns 124, first conductive patterns 126 and second conductive patterns 128, respectively.
Hereinbelow, the operation of timer 54a having the above structure will be described.
If a user rotates timer knob 102 in a clockwise direction, first, second and third contacts 120a, 120b and 120c of contact arm 120 are moved in a clockwise direction from at an original position of contact stop 130 to a following position of contact stop 130. First and second contacts 120a and 120b are brought into contact with pulse generating conducts 126a and 28a during the movement of contact arm 120. Such contact operations of first and second contacts 126a and 128b generate pulse signals. The number of pulses of the individual pulse signal depends on the rotating angle of timer knob 102. Microprocessor 20 judges the amount of angular displacement of timer knob 102 in a clockwise direction based on the generated pulse signals, and determines the setting period of timer 54a. That is, microprocessor 20 determines the setting period of timer 54a based on the displacement of contact arm 120 according to the rotation of timer knob 102.
On the other hand, if the user rotates timer knob 102 in a counter-clockwise direction, pulse signals are generated based on an operational principle such as the user rotating timer knob 102 in a clockwise direction. Microprocessor 20 determines the setting period of timer 54a based on the displacement of contact arm 120 according to the rotation of timer knob 102 in the same manner that the user rotates timer knob 102 in a clockwise direction.
As described above, Masayuki Aoki's timer 54a determines the setting period of the timing sections based on the rotating direction of timer knob 102 and pulse signals generated by contacting the electric contacts with the conductive patterns. That is, timer 54a is capable of setting the cooking period and performing a cooking start by using single timer knob 102. However, Masayuki Aoki's timer 54a has a plurality of complicated conduct patterns on switch substrate 112 and a plurality of internal wires for adding the cooking start function. Consequently, the Masayuki Aoki's timer 54a is disadvantageous in that the internal structure is complicated and the manufacturing cost is high.